Examining a supermassive black hole with the help of gravity

Thanks to a serendipitously placed galaxy, astronomers have been able to use …

General relativity describes how gravity is the result of the warping of spacetime by mass-energy. When this concept is applied to the flight path of a photon, which, in the absence of external effects, travels in a perfectly straight line, you find that photons travel along a geodesic—a straight line in curved space. This can lead to the non-intuitive effect known as gravitational lensing, where a far-off object can be magnified or distorted by a large mass between it and the observer.

The Einstein Cross
Image Credit: ESO/F. Courbin et al.

One famous example of gravitationallensing is Einstein's cross (QSO2237+0305). It is a single quasar that exists nearly 8 billion light-years from Earth. But, due to a galaxy only 400 million light years from here in the line of sight of the quasar, it appears as four distinct images. The apparent multitude of objects is due entirely to the lensing caused by the mass of the intervening galaxy.

Astronomers from the US, Switzerland, and Germany have been observing the Einstein Cross with the ESO's Very Large Telescope (VLT) for the past few years. The configuration of the quasar and the intermediate galaxy results in what is termed macrolensing, where the galaxy acts as a cosmic magnifying glass, allowing humans to see the otherwise unobservable quasar. The individual stars within the lensing galaxy result in what is known as microlensing; here the images do not change, but the wavelength and brightness of the light that reaches Earth gets distorted.

In a paper published in Astronomy and Astrophysics, the team reports repeated measurements of the color and intensity of the light from each of the four images of the quasar to probe the nature of its supermassive black hole and the accretion disk that power the quasar. Using the data, they were able to derive the energy profile of the accretion disk. They find that it is well described by a power law (e.g. R∝λζ) with ζ equal to 1.2±0.3. This result agrees well with both the standard thin accretion disk model (ζ=4/3) and a model that assumes the accretion disk is powered by the rotation of the black hole (ζ=8/7).

In addition to carrying out the first ever direct measurement of a quasar accretion disk, the technique illustrated how small a volume of space that could be resolved. Using the combination of macro- and micro-lensing along with the VLT, the astronomers were able to discriminate areas as small as 0.1 microarcseconds, over 10,000 times better than the resolution of the best telescopes in the world. To put this in perspective, this implies that this technique could see an object that is between the size of a nickel and a quarter at a distance of around 5 million kilometers, about 13 times further away from Earth than the moon is.

Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm